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Difference between revisions of "Metal reclamation and recycling of electronic waste"

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===Bioleaching===
 
===Bioleaching===
Bioleaching is a fairly new process of using bacteria and fungi to separate metals from electronic waste. It promises to be very energy efficient. Organisms such as ''Bacillus'' sp., ''Saccharomyces cereÍisiae'', and ''Yarrowia lipolytica'' leach lead, copper, and tin from printed circuit boards when shredded into sub-milimeter sizes. Under ideal conditions, ''T. ferrooxidans'' and ''T. thiooxidans'' were able to mobilize at least 90% of the aluminum, copper, nickel and zinc present <ref>Brandl, H. et Al., 1999, "Computer-munching microbes: metal leaching from electronic scrap by bacteria and fungi," Hydrometallurgy, '''2001''', (59).</ref>. One type of bacteria, C. violaceum, was able to leach gold from larger pieces of electronic waste (5 x 10 mm). It disolved 14.9% of the approximately 10 mg of gold present as dicyanoaurate <ref>Cui, J. et Al., 2007, "Metallurgical recovery of metals from electronic waste: A review," Journal of Hazardous Materials, '''2008''', (158), p. 18.</ref>.
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Bioleaching is the process of using bacteria and fungi to separate metals from electronic waste. It promises to be very energy efficient. Organisms such as ''Bacillus'' sp., ''Saccharomyces cereÍisiae'', and ''Yarrowia lipolytica'' leach lead, copper, and tin from printed circuit boards when shredded into sub-milimeter sizes. Under ideal conditions, ''T. ferrooxidans'' and ''T. thiooxidans'' were able to mobilize at least 90% of the aluminum, copper, nickel and zinc present <ref>Brandl, H. et Al., 1999, "Computer-munching microbes: metal leaching from electronic scrap by bacteria and fungi," Hydrometallurgy, '''2001''', (59).</ref>. One type of bacteria, C. violaceum, was able to leach gold from larger pieces of electronic waste (5 x 10 mm). It disolved 14.9% of the approximately 10 mg of gold present as dicyanoaurate <ref>Cui, J. et Al., 2007, "Metallurgical recovery of metals from electronic waste: A review," Journal of Hazardous Materials, '''2008''', (158), p. 18.</ref>.
  
 
== Progress towards a sustainable future ==
 
== Progress towards a sustainable future ==

Revision as of 04:47, 15 November 2008

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Currently, the majority of electronic waste is disposed of in a landfill. The remainder is either processed in energy inefficient method that only recovers a small amount of the available metals or burned to recover saleable metals such as copper, aluminum and iron. This is a problem for a number of reasons.

  • This waste contains recoverable trace amounts of precious metals, and larger quantities of a variety of other metals and alloys, especially copper, aluminum and steel.
  • Electronic waste contains high concentrations of heavy metals, brominated flame retardants and other plastic additives that have proven adverse effects on humans. Note: more information can be found at Electronic waste.
  • The volume of electronic waste produced is high, and growing fast [1]. In 2005, the United States alone disposed of was at least 1.5 million tons of electronic waste, and at most one quarter of that was recycled [2]. The situation is similar in Canada, with more than 71 000 tonnes of waste being disposed of in 2005, with just 26% being recycled [3]. In the province of Ontario, Canada, the percentage recycled in 2004 was just 2% [4].
  • Discarding recoverable metals is energy inefficient. The energy savings possible by recovering metals are high compared to mining new material. The energy savings for aluminum is 95%, copper 85%, steel 74%, lead 65%, and zinc 60%. Recovering other materials such and paper and plastics can reduce energy usages by 64% and 80%, respectively [5]. The worst case scenario eliminates only the ore extraction and transportation steps, which are both energy intensive.

Materials involved

There are a wide variety of materials involved in the multitude of items that can be classified as electronic waste. Each component adds to the complexity of any recycling effort. Below is an exploration of a number of the most common components, by weight.

Printed circuit boards

A printed circuit board, or PCB, is the piece of hardware acts as a base and provides electrical connections to the mounted components. They are present in many types of electronic waste, including cellphones, computers, TVs, and printers. A PCB is made of a number of components. Each step of the productions process is outlined below, to give an idea of the variety of materials involved.

picture of a circuit board here

FR-4 is the most common base material [6] for printed FR-4 is an abbreviation of Flame Retardant 4, referring to its flame resistance and self-extinguishing properties. It is a brittle material formed by hardening a woven fiberglass sheet with an epoxy resin, usually created from ethylene clorohydrin and bisphenol-A [7]. To give it the aforementioned self extinguishing properties, a brominated flame retardant is incorporated in the epoxy. Some flame retardants can be incorporated at the molecular level, like TBBPADEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. [8]. It can be easily coloured, with common colours including green, blue, red, and black.

The type glass used to create the fiberglass sheets is S-glass. Content ranges, by weight are, 52-56% silicon dioxide, 16-25% calcium oxide, 12-16% aluminum oxide, 5-10% boron oxide, 0-2% sodium oxide or potassium oxide, 0-5% magnesium oxide, 0.05-0.4% iron oxide, 0-0.8% titanium oxide, and 0-1% fluorides [9].

The FR-4 is layered with copper traces, and bonded with subsequent layers. Computer motherboards today have as many as eight layers [10].

Surface mounted components

A wide variety of components are soldered to printed circuit boards. A resistor is comprised of copper leads attached to a painted ceramic or carbon core [11]. Microchips are composed of small amounts of silicon, aluminum, and copper, [12] with plastic coatings. CPUs today have aluminum heatsinks as well.

Casings

Most consumer electronic devices have plastic casings, such as a TV or a cell phone. Household appliances also have aluminum or steel cases. Other products, such as computer cases have both metallic and plastic components to them.

CRT and LCD screens

CRT, or cathode ray tubeDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD., monitors are currently being phased out in favour of LCDDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. monitors.

A CRT monitor consists of the electron gun and focusing equipment, a plastic casing and leaded glass. An analysis of the glass resulted in the following results, by weight, are 49.61% silicon dioxide, 24.17% lead oxide, 7.79% potasium oxide, 5.32% sodium oxide, 3.63% aluminum oxide, 2.99% strontium oxide, 2.30% calcium oxide, 1.96% barium oxide, 1.49% magnesium oxide, 0.58% zirconium oxide, 0.07% iron oxide and 0.07% phosphorous oxide [13]. The lead and phosphorous oxide [14] are most difficult to deal with. Most of the lead (up to 98.6%) can be recovered through a pyrovacuum process, where high heat and low pressure allow the lead to be vaporized and later recovered, with the aid of carbon powder[15].

Wire

Wire is a common element in electronic waste, as it has a method of connecting to a household wall socket, either through a power cord or charger. Internal wiring is common especially in desktop computers and older audio and video equipment.

Batteries

Batteries are a staple for portable electronic devices, whether in lithium ionDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD., rechargable NiCdDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. or NiMHDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD., AAA or AA alkaline cellDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. form. While batteries are electronic waste, they will not be covered in this analysis. There are numerous battery recycling programs; see [1][2][3].

Precious metals

Precious metals are used in electronics for their superior conductivity and resistance to oxidation. They are used as A study undertaken by Cui, J. et Al., at the Norwegian University of Science and Technology showed that there are recoverable amounts of precious metals, such as silver, gold and palladium. For typical electronic scrap, they found and average of 2000 ppm silver, 1000 ppm gold and 50 ppm palladium. Outliers include printed circuit boards with 3300 pm silver and cell phones with 210 ppm palladium[16]. This is significant, as this means that in the United States, using 2005 data [17], these numbers show that there was up to 1500 tons of gold and 3000 tons of silver in the electronic waste that was disposed of in the U.S.A. in 2005.

Current solutions

Solutions is a misnomer for this category. Electronic waste processed in a manor shown below either does not recover usable materials, is selective of feedstock, or processes the electronic waste in a manner that has measured environmental effects.

Shredding

In this process, commonly known as mechanical e-waste recycling, electronic waste is shredded by specialized equipment. An example of a shredder used in Kansas, U.S.A.[18] may be found here. This waste is then sorted mechanically, by magnetic fieldDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. or eddy currentDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. separators, or novel means such as vertical vibration separation[19]. This last method has demonstrated effective for separating metals from plastics, especially copper[20].

Another product of this method is fine dust. Research has been conducted into possible uses of this dust. In a study conducted by Kakimoto, K et Al., it can be successfully incorporated into Portland cement, without the loss of strength, up to %30 by weight. An analysis of this dust found to that it was mainly silicon dioxide, calcium oxide and aluminum oxide, with trace amounts of lead and copper.

This process is effective at separating metals from plastics, but fails to address the separation of low concentration metals from printed circuit boards.

Municipal incineration

Municipal incineration is simply just incinerating electronic waste with other household and business waste in a municipal incinerator. While many components of electronic waste have high usable energy content, this dilutes metal concentrations down even further.

Stewart, E. et Al. studied the emissions of incinerated computer components and monitored it for VOCs, SVOCs, dioxins, halogens, and metals. With a mass flow rate of 1.944 kg/h, their findings are as follows, with all weights listed being per dry standard cubic meter. The halogen chlorine at 7.9 mg/dscm, dichlorine 0.3 mg/dscm, bromine 13.1 mg/dscm, dibromine 17.8 mg/dscm. The metal concentrations were copper at 4600-7950 µg/dscm, lead 3790-4840 µg/dscm, antimony 790-1540 µg/dscm, cadmium 55-81 µg/dscm, manganese 19-52 µg/dscm, nickel 8-17 µg/dscm, barium 6-28 µg/dscm, arsenic 3-6 µg/dscm, chromium 3-4 µg/dscm, cobalt 0.7-2.3 µg/dscm, and beryllium 0.1-0.3 µg/dscm. Volatile organic compounds were present at bromobenzene 830-920 µg/dscm, tribromomethane 40-70 µg/dscm, bromomethane 20-190 µg/dscm, benzene 25-35 µg/dscm, dibromomethane 0-20 µg/dscm. Semi-volatile organic components were measured at naphtalene 1.2-3.3 µg/dscm, acetophenone 1.0-2.0 µg/dscm, chlorobenzene 0.4-1.4 µg/dscm, and dibenzofuran 0.3-0.8 µg/dscm [21]. These levels are all extremely small. It would be easy to dismiss them as insignificant, except for the scale of the trial - at less than 2 kg per hour, scaling up to multiple tonnes as hour would cause a proportionate increase in emissions. An output of such a wide range of harmful substances would then disqualify this method as a sustainable way of dealing with electronic waste.

Pyrometallurgical recovery

This process is conducted by adding shredded electronic waste with high copper contents to copper ore concentrate and is then refined through the use of heat. Two examples of this, each recycling about 100 000 tonnes of electronic waste per year, are shown below.

At a copper smelter, in Quebec, Canada, the Noranda process is used. Copper ore concentrates at about 24% copper and shredded electronic waste are immersed in a liquid metal bath, at 1250 C. Oxygen enriched air (39% oxygen) is added. This converts iron, lead and zinc into oxides, which are removed with the silicon dioxide based slag. It is refined in an anode furnace, resulting in an alloy of 99.1% copper, with the remainder made up of silver, gold, platinum, palladium, selenium, tellurium and nickel [22].

At the Ronnskar smelter in Sweden, a similar process is undertaken. Shredded electronic waste with low metal concentrations is added at the start of the process, while higher concentration e-waste is added later. Zinc is removed at an earlier step with slag and then separately refined. During the refining process, precious metals such as selenium, gold, silver and palladium, are removed at seperate steps than lead, and nickel. This leaves a high purity copper as the final product [23].

This process is relatively efficient, but is only useful for shredded electronic waste that meets the requirement of high metal content. Also, this process can not deal with aluminum, not does it acount for the dioxins created due to the presence of brominated flame retardants[24].

Open flame incineration

The waste that is shipped to India, China and other Asian countries is burned, cut up, or dissolved in acids to recover metals, mainly copper that can be sold to scrap dealers. Components that have no resale value are dumped. Numerous written [4] and video accounts [5][6] are available.

Proposed solutions

Below are detailed the proposed solutions. It should be noted that none of them are complete solutions to the current problem.

Thermal depolymerization

Thermal depolymerization is a process in which thermal energy, under high pressure conditions and with the aid of water, is used to decompose organic molecules. No other solution It would, in theory, render plastics and epoxies present into usable oil. The resulting solids would have much higher concentrations of metals.

Plasma arc gasification

Bioleaching

Bioleaching is the process of using bacteria and fungi to separate metals from electronic waste. It promises to be very energy efficient. Organisms such as Bacillus sp., Saccharomyces cereÍisiae, and Yarrowia lipolytica leach lead, copper, and tin from printed circuit boards when shredded into sub-milimeter sizes. Under ideal conditions, T. ferrooxidans and T. thiooxidans were able to mobilize at least 90% of the aluminum, copper, nickel and zinc present [25]. One type of bacteria, C. violaceum, was able to leach gold from larger pieces of electronic waste (5 x 10 mm). It disolved 14.9% of the approximately 10 mg of gold present as dicyanoaurate [26].

Progress towards a sustainable future

Alternative materials and manufacturing

A number of alternatives have been proposed to the materials and processes currently used. These initiatives are detailed below.

  • An alternative to FR-4 is a circuit board composed of chicken feathers with a soy-based epoxy. If this technology becomes commonplace, this would have a massive effect on the composition of the average electronic waste.
  • Lead free solder. The RoHSDEPRECATED TEMPLATE - PLEASE USE {{W}} INSTEAD. Directive of the European Union currently specifies that products that are sold in their jurisdiction must contain no more than trace amounts of lead. Fortunately, this has had an impact on products availible in North America, with the RoHS logo adorning many of the relevant electronic products in N.A. as well. This, along with the move to LCD based monitors and TVs will cause a large reduction in percentage lead composition of electronic waste.
  • Elimination of halogens. Simply by reading this page, the complications that halogens such as bromine cause are made apparent. An alternative to brominated flame retardants would have a very positive effect on the environmental release of halogens due to electronic waste.

Comprehensive processing plan

From all of the processes outlined so far, an attempt at a comprehensive plan for the recovery of metals and ultimately the recycling of electronic waste can be made.

References

  1. http://www.epa.gov/epawaste/conserve/materials/ecycling/manage.htm Accessed Nov. 11, 2008
  2. http://www.epa.gov/epawaste/conserve/materials/ecycling/docs/fact7-08.pdf Accessed Nov. 11, 2008
  3. http://www.ec.gc.ca/wmd-dgd/default.asp?lang=En&n=F3852FB1-1 Accessed Nov. 11, 2008.
  4. http://www.ene.gov.on.ca/en/news/2007/061201.php
  5. Nnorom, I. et Al., 2007, "Overview of electronic waste (e-waste) management practices and legislations, and their poor applications in the developing countries," Resources, Conservation and Recycling, 2008, (52), p. 5.
  6. Coombs, C., 2001, Printed Circuits Handbook Fifth Edition, McGraw-Hill, New York, section 6.
  7. http://www.p-m-services.co.uk/how%27s_fr4_made_.htm Accessed Nov. 7, 2008.
  8. Coombs, C., 2001, Printed Circuits Handbook Fifth Edition, McGraw-Hill, New York, section 6.4.2 and 6.2.3.
  9. Coombs, C., 2001, Printed Circuits Handbook Fifth Edition, McGraw-Hill, New York, section 6.5.1
  10. http://www.fudzilla.com/index.php?Itemid=37&id=7256&option=com_content&task=view Accessed Nov. 7, 2008.
  11. http://www.ecawa.asn.au/home/jfuller/electronics/resistors.htm Accessed Nov. 11, 2008.
  12. http://www.intel.com/education/makingchips/index.htm Accessed Nov. 11, 2008.
  13. Chen, M. et Al., 2008, "Lead recovery and the feasibility of foam glass production from funnel glass of dismantled cathode ray tube through pyrovacuum process," Journal of Hazardous Materials, currently unprinted, p. 2.
  14. http://avogadro.chem.iastate.edu/MSDS/P2O5.htm Accessed Nov. 12, 2008.
  15. Chen, M. et Al., 2008, "Lead recovery and the feasibility of foam glass production from funnel glass of dismantled cathode ray tube through pyrovacuum process," Journal of Hazardous Materials, currently unprinted, pp. 1-5.
  16. Cui, J. et Al., 2007, "Metallurgical recovery of metals from electronic waste: A review," Journal of Hazardous Materials, 2008, (158), p. 3.
  17. http://www.epa.gov/epawaste/conserve/materials/ecycling/docs/fact7-08.pdf p. 1, Accessed Nov. 11, 2008
  18. http://www.prlog.org/10046578-automated-waste-shredding-system-operational.html Accessed Nov. 12, 2008.
  19. Mohabuth, N. et Al., 2006, "Investigating the use of vertical vibration to recover metal from electrical and electronic waste," Minerals Engineering, 2007, (20).
  20. Mohabuth, N. et Al., 2006, "Investigating the use of vertical vibration to recover metal from electrical and electronic waste," Minerals Engineering, 2007, (20), pp. 3-6.
  21. Stuart, E. et Al., 2003, "Emissions from the Incineration of Electronics Industry Waste," IEEE, 2003, (03).
  22. Cui, J. et Al., 2007, "Metallurgical recovery of metals from electronic waste: A review," Journal of Hazardous Materials, 2008, (158), p. 5.
  23. Cui, J. et Al., 2007, "Metallurgical recovery of metals from electronic waste: A review," Journal of Hazardous Materials, 2008, (158), p. 6.
  24. Cui, J. et Al., 2007, "Metallurgical recovery of metals from electronic waste: A review," Journal of Hazardous Materials, 2008, (158), p. 8.
  25. Brandl, H. et Al., 1999, "Computer-munching microbes: metal leaching from electronic scrap by bacteria and fungi," Hydrometallurgy, 2001, (59).
  26. Cui, J. et Al., 2007, "Metallurgical recovery of metals from electronic waste: A review," Journal of Hazardous Materials, 2008, (158), p. 18.